The present invention relates in general to measuring the thickness of a thin layer in a multilayer structure using ultrasound, and more specifically to a tone-burst method for resolving an imbedded layer in a structure having high attenuation of ultrasonic pulses.
Polyethylene fuel tanks are used in automotive vehicles because of their light weight, durability, ease of molding into complex shapes, and corrosion resistance. Due to the high level of hydrocarbon permeation of polyethylene, however, hydrocarbon vapors could escape from a polyethylene fuel tank. Therefore, a barrier layer of a different polymer is normally incorporated into the tank wall by co-extrusion of the polyethylene and the barrier layer. The barrier layer is typically embedded between layers of polyethylene in order to provide structural protection of the barrier layer.
The thickness of various layers and, in particular, the presence and thickness of the barrier layer must be monitored during manufacture of the fuel tanks. Extrusion machines must be adjusted to provide proper thickness prior to a production run of tanks and then thickness must be monitored during production for purposes of quality assurance.
Destructive testing of sample tanks whereby a tank wall is cross sectioned and visually inspected to measure barrier layer thickness is undesirable due to the scrap created and the time required to conduct such a test. Therefore, a non-destructive and fast technique such as using ultrasound is preferred.
Conventional pulse-echo ultrasound is capable of measuring the thickness of outer and inner polyethylene layers as well as detecting the presence of a barrier layer. The conventional technique, however, is incapable of measuring the thickness of the barrier layer due to insufficient resolution.